Saturday, May 10, 2008

Ammonia Function on Pyrophosphate Plating

A small amount of ammonia is used to produce more uniform and lustrous deposits and to improve anode corrosion. Excess ammonia can cause suprous oxide, 140 g of ammonium hydroxide per square meter of exposed surface per day, or as determinate by chemical analysis of the bath. In some installations, gaseous ammonia is used. Ammonia consumption will increase with increasing temperature, pH, agitation and ammonia content in the bath.

Current had little effect on consumption. OrthophosphateOrthophosphate formed by hydrolysis of pyrophosphate is beneficial in that it promotes anode corrosion and acts as a buffer. It has no harmful effects on the plating characteristics of a bath until its concentration reaches about 100 g./l. Above this concentration, conductivity and bright plating range are decreased and banded deposits are obtained.Low pH (less than 7), high P2O7/Cu ratio, high temperature (greater than 60oC) in the plating bath or heat exchanger, or local overheating will cause orthophosphate to build up. Orthophosphate can't be removed only by either discharging the bath altogether or by diluting and rebuilding it.

When use at controlled, limited concentrations, organic additives refine the grain structure, impart leveling characteristics to the plating bath, and act as brighteners. However, decomposition products from an excessive additive concentration can cause brittle copper deposits. Thus for optimum quality deposits, no more additive should be added to a bath than is necessary to replace losses by consumption.Organic additives covered in the patent literature as brighteners include mercaptothiadiazoles, mercaptothiazoles, mercaptobenzimidiazole and pyrimidines.

Numerous other materials such as glyserol, triethanolamine, trioxyglutaric acid, diphenylamine sulfonic acid, napthalene disulfonic acid, gelatine, bakers yeast, casein, glycocoll, sodium selenite, sodium sulfite, Rochelle salt, sodium thiosulphate, potassium bromide, and the chlorides of As, Bi, Fe, Cr, Sn, Zn, Cd, Pb and alkali metals have been used as brighteners. Addition of small amounts of lead as lead carbonate are reported to improve plating properties. Organic acids such as oxalic, lactic, tartaric, malic, and citric or their ammonium or alkali salts also produce some brightening.